I do not think that there is a time persay, but it depends on what the market will stand
I was also a builder prior to RE
My time as a builder was before the explosion of the contract tradie, I employed tradesmen on wages
Also as a traditional builder I did the majority of the work myself, 5 months per house.
Now you need a plethora of tradespeople to build a house as every tradesman ( Loosely used word) is only capable of doing one single job
This change has increased costs dramatically and lowered standards dramatically
I remember talking to an elderly builder in Qld (before it boomed) and we posed a similar question
His answer
I had X in the bank when I started and I built this house and had X in the bank when I finished so I took the cost from that and added a "bit" of profit and that is what I charged the buyer !! and he was happy with that deal